Types of Congenital Disorders

Chromosomal Abnormalities

One of the most common types of birth defects is a chromosome condition, which can result in physical problems, intellectual disability (formerly known as mental retardation), or both. Chromosomes are structures found in the center (the nucleus) of cells and contain our genetic material, or genes. This material is the building block of the human body. Typically, humans have 46 chromosomes in every cell; these 46 chromosomes are arranged into 23 pairs, with one in each pair coming from the mother’s egg and the other coming from the father’s sperm. Males have one X and one Y chromosome and females have two X chromosomes.

Chromosome conditions occur when there is variation in either the total number or in the structure of the chromosomes. While typically we receive 23 chromosomes from each parent, on occasion there may be an extra or a missing chromosome in either the egg or the sperm that then gets passed on to the developing fetus after fertilization. The most well-known and common condition with a difference in chromosome number is Down syndrome (also known as trisomy 21). Down syndrome occurs when there are three, instead of two, copies of chromosome 21 in every cell in the body, for a total of 47 chromosomes.

There are other conditions besides Down syndrome where an extra chromosome is present. Trisomy 18 (Edwards syndrome) occurs when there are three copies of chromosome 18 in every cell and trisomy 13 (Patau syndrome) occurs when there is an extra chromsome 13 in every cell. These conditions are more rare than Down syndrome. While conditions in which there is a chromosome missing tend to be less common than ones in which there is an extra chromosome, Turner syndrome is a well-known condition in which a girl/woman is missing one of her two X chromosomes.

There is always a small risk for a chromosome condition for a woman regardless of her age. While research has shown that the risk does increase as a woman gets older, it is overall still low.